Cathodic protection current distribution method and apparatus for corrosion control of reinforcing steel in concrete structures

ABSTRACT

Mixed-metal-oxide (MMO) coated precious-metal tape is installed directly on concrete surfaces using an electrically non-conductive adhesive with semi-conductive coating or overlay, thereby obviating the need for slots or holes to the concrete structures which are not subject to direct moisture. The tape anodes may be installed on the concrete surfaces exposing metals without developing an electrical short circuit between the anode and the metals due to the non-conductive adhesive. The semi-conductive layer over the metal tape can distribute the CP current uniformly to the entire concrete surface without developing electrical short circuit. Overall the invention provides for quick and low cost installation on many concrete structures. Interconnections between the tape anodes and bare metal distribution elements may be made with conductive adhesive or spot welding.

FIELD OF THE INVENTION

This invention relates generally to corrosion control inreinforced-concrete structures and, in particular, to mixed-metal-oxide(MMO) coated precious-metal tape and mesh that may be installed directlyon concrete surfaces without the need for slots, holes, cementitiousgrout or concrete.

BACKGROUND OF THE INVENTION

Cathodic protection (CP) is a method for controlling corrosion ofreinforcing steel, including steel structures in chloride contaminatedconcrete. Various types of impressed current cathodic protection anodesfor reinforced concrete structures have been developed in the past. Theanode is one of the most critical components for a cathodic protectionsystem, as it is used to distribute cathodic protection current to thereinforcing steel.

One of the most effective and durable anodes is made of a material whichis resistance to corrosion, for example a mixed-metal-oxide (MMO) coatedtitanium substrate. MMO coated anodes are manufactured by coating amixture of precious metal oxides on a specially treated precious metal.The coated substrate undergoes multiple thermal treatments at elevatedtemperatures to achieve good bonding properties between the substrateand the coating. Although titanium is widely used as substrate materialdue to its resistance to corrosion, resistance to chemical attacks andhigh mechanical strength, other anodes such as tantalum, niobium andzirconium anodes are also used for different applications.

Since the first MMO-coated titanium anode was developed in 1984, manyconcrete structures have been protected using this material. To installthe anodes, however, they must be embedded in concrete or cementitiousgrout. For example, titanium mesh with a concrete overlay, titaniumribbon or ribbon mesh embedded in cemetitious grout in saw-cut slots, ordiscrete anodes embedded in grout in drilled holes. However, these typesof installations add burden to the structure and lead to some durabilityconcerns. A useful review of MMO-coated anodes and installationtechniques may be found in “Cathodic Protection of Steel in Concrete” ByPaul Chess, Taylor & Francis (1998), ISBN 0419230106, the entire contentof which is incorporated herein by reference.

For the slotted or discrete types of installations, the existingconcrete must be cut or drilled to install the anodes. However, when theconcrete covers over the reinforcing steel are shallow or congested,such installation procedures are not feasible. Even if the anodes aresomehow installed in the slots or drilled holes, the vicinity of thereinforcing steel near the anodes may cause an electrical short circuit,resulting in malfunction of the cathodic protection system.

When a MMO-coated titanium anode is operated greater than 110 mA/m² ofanode current density in chloride contaminated concrete, acid isgenerated at the anode-concrete interface due to the chlorine gasevolution by the anodic reaction. As a result, cement paste of theconcrete as the electrolyte which contact to the anode is dissolved bythe acid. This leaves the non-conductive aggregates at theanode-concrete interface and causes the increases of the circuitresistance, diminishing the cathodic protection current.

When MMO coated precious metal tape is installed with electricallyconductive adhesive, or when any form of MMO coated precious metalanodes are embedded in the dry concrete structure which is not subjectto direct moisture, rain or seawater splashes, the circuit resistanceincreases with time due to the electrochemical osmosis at theanode-concrete interface. Once the circuit resistance exceeds themaximum DC power supply, the current from the anodes decrease with time.The electrochemical osmosis condition increases with increasing theanode voltage. Eventually, the anodes cannot discharge any current atthe maximum voltage of the power supply.

When high conductive media is used as an anode to cover the concretesurface, an electrical short circuit is often developed by the exposedmetals, such as rebar chairs, steel wire ties. When highly conductivemedia is used as an anode to cover the concrete surface, theconcentration of cathodic protection current from the local portion ofthe anode system to shallow rebars develop acid generation, resulting inpoor current distribution to a concrete structure.

SUMMARY OF THE INVENTION

This invention overcomes the shortcomings of prior art by allowingmixed-metal-oxide (MMO) coated precious-metal tape to be installed ondry concrete structures without the need for slots or holes. In thepreferred embodiments, a semi-electrically conductive and gas-permeablecoating or overlay is used in combination with a MMO-coatedprecious-metal tape anode to improve current distribution at lowvoltages. The electrical resistivity of the semi-conductive coating islower than that of typical concrete. However, when the semi-conductivecoating is moist in a local area, the resistivity is still high enoughto prevent the concentration of cathodic protection current dischargingto the concrete substrate.

According to the invention, MMO-coated tape anodes may be installed onthe concrete surfaces using non-conductive adhesives without developingan electrical short circuit between the anode and the reinforcing steel.Interconnections between the tape anodes and bare metal distributionelements may be made with conductive adhesive or spot welding.Alternatively, MMO-coated mesh anodes may be embedded in thesemi-conductive coating without contacting to the concrete substrate.Interconnections between the mesh anodes and bare metal distributionelements may be made with conductive adhesives or spot welding.

Since MMO coated anodes are not embedded or contact to concrete, theanodic can be operated at anode current densities higher than 110 mA/m².The chlorine gas evolved on the anode diffuses away though the poroussemi-conductive coating before it tunes to hydro-chloric acid. Byutilizing this high current discharge capability of the anode systemwith the semi-conductive coating, the system facilitateselectro-chemical chloride extraction from the chloride contaminatedconcrete.

Through the use of a semi-conductive layer covering over the anodes,electrical short circuits can be prevented even though thesemi-conductive coating contacts with exposed metals from the concretesurface. The semi-conductive media may include carbon, MMO coatedmetal(s) or any passive bare metal powder or fibers mixed with anyelectrolytic cementitious or plastic media.

When carbon powder or fibers are used as the electrical media to producesemi-conductive layer, the carbon is consumed by passing through thecathodic protection current with time. However, when a large current isrequired for a long time of period, MMO coated metal powder or fibersmay be used to extend the life of the semi-conductive layer. Whenpassive metal powder or fibers are used under their break-downpotential, they can be used without consumption of the metals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates tape anode installation on a concrete surface andcovered with a semi-conductive coating. The tape anode is fixed on theconcrete surface by non-conductive adhesive and the top of the tapedischarge current;

FIG. 2 illustrates a mesh anode installation within a semi-conductivecoating;

FIG. 3 is an example of a possible installation method of tape anodes toa bare-metal element; and

FIG. 4 illustrates chlorine gas generated on an anode diffusing away toatmosphere.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to protection and corrosion prevention ofreinforced concrete structures using mixed-metal-oxide (MMO) coatedprecious-metal tape anodes. In the preferred embodiments, the anodes areattached to a concrete surface using a non-conductive adhesive andcovered with a semi-conductive layer to provide cathodic protection orchloride removal. As such, the bare metal surface of the tape is fixedon the concrete surface using a non-conductive adhesive, such that theMMO-coated metal surface of the tape is exposed.

The substrate metal tape anode may be composed of titanium, tantalum,zirconium, or niobium. However, the most preferred metals are titaniumor titanium alloys because of the corrosion resistance and availability.The tape anode width is preferably over 5 mm, and the thickness is inthe range of 0.001 mm to 1 mm, preferably between 0.1 mm to 0.3 mm.

FIG. 1 is a simplified cross-sectional diagram showing a metal tapeanode 1 attached to concrete 4 through non-conductive adhesive 3. Todistribute cathodic protection current to the entire concrete surfacebefore traveling into the concrete and reinforcing steel bars, thincoated semi-conductive media 2 is coated over the metal tapes.

The semi-conductive coating preferably comprises a cementitious orplastic material with suspended conductive or semi-conductive particlesto adjust the electrical resistivity. In accordance with one preferredembodiment, the semi-conductive coating comprises a flowable, hardeningcementitious or plastic medium, which may include a layer of carbonfibers or passive metal fibers, further including a distribution ofoxides of titanium, tantalum, iridium, ruthenium, palladium, or cobalt.By adjusting the composition of the semi-conductive media 2, theelectrical resistivity may range from 100 ohm-cm to 20,000 ohm-cm. Thesemoderately high resistances prevent electrical short circuits if anymetal exposed from the concrete. However, the electrical resistivity islow enough to distribute the current to the entire covering concretesurface. The thickness of the semi-conductive media 2 is in the range of1 mm to 25 mm, preferably between 3 mm to 7 mm.

Furthermore, as shown in FIG. 2, the anode tapes are typically spaced onconcrete surfaces according to the cathodic protection currentrequirement for the reinforcing steel in concrete. The spacing is alsobased on the current requirement of the reinforcing steel. As shown inFIG. 3, the tape anodes 1 or 6 may be electrically interconnected atpoints 8 to bare metal tapes 7 by means of spot welding or conductiveadhesive. The “bare” metal tape may be the same metal as the tape anodeor different materials may be used.

The semi-conductive coating is resistant to the acid which may developon the anode surface. FIG. 4 illustrates how chlorine gas evolved on themetal tape surface and within the semi-conductive coating may diffuseaway from the anode system though the porous coating 2. Because theinvention allows the anodes to operate at anodic current densitieshigher than 110 mA/m² without generating acid, this also allows using aschloride removal system using larger current densities. The negativelycharged chlorides which are attractive to the positive charged anode,they turn to chlorine gas. The gas diffuses away to the surroundingatmosphere through the semi-conductive layer.

I claim:
 1. A system for controlling the corrosion of reinforcing steelin a concrete body having a surface, comprising: a mixed-metal-oxide(MMO) coated precious-metal anode indirectly bonded to the surface ofthe concrete body through an electrically non-conductive material; and asemi-conductive coating covering the anode and at least portions of thesurface of the concrete body to distribute cathodic protection currentfrom the concrete surface to the anode without the anode making a directelectrical connection to the concrete body.
 2. The system of claim 1,wherein the electrically non-conductive material is a non-conductiveadhesive.
 3. The anode of claim 1, wherein the anode is composed ofcarbon, titanium, tantalum, zirconium, niobium, or alloys thereof, orMMO coated titanium, tantalum, zirconium, niobium, or alloys thereof. 4.The anode of claim 1, wherein the coating is composed of oxides oftitanium, tantalum, iridium, ruthenium, palladium, or cobalt.
 5. Thesystem of claim 1, wherein the anode is in the form of an elongate tape.6. The system of claim 1, wherein the anode is in the form of anelongate tape having a width of 5 mm or greater and the thickness in therange of 0.001 mm to 1 mm.
 7. The system of claim 1, including aplurality of anodes interconnected with a bare metal tape.
 8. The systemof claim 1, including a plurality of interconnected anodes spaced-aparton the surface of the concrete.
 9. The system of claim 1, including aplurality of anodes interconnected with a bare metal tape using anelectrically conductive adhesive.
 10. The system of claim 1, including aplurality of anodes interconnected to a bare metal tape usingspot-welding.
 11. The system of claim 1, wherein the semi-conductivecoating includes a layer of carbon fibers or passive metal fibers. 12.The system of claim 1, wherein the electrical resistivity of thesemi-conductive coating is in the range of 100 ohm-cm to 20,000 ohm-cm.13. The system of claim 1, wherein the semi-conductive coating issufficient porous for chlorine gas escape therethrough to atmosphere.14. The system of claim 1, wherein the anode is in the form of a mesh.